Sensor properties of hybrid SnO2-polysilazane materials

Procedia Chemistry Procedia Chemistry 1 (2009) 172–175 www.elsevier.com/locate/procedia

Proceedings of the Eurosensors XXIII conference

Sensor properties of hybrid SnO2-polysilazane materials E.A. Makeevaa*, M.N. Rumyantsevaa, A.M. Gaskova, B.A. Ismailovb, V.A. Vasnevc a Chemistry Department, Moscow State University, Leninskiye Gory 1-3, Moscow 119991, Russia Moscow State Textile University 'A.N. Kosygin', Malaja Kaluzskaja str. 1, Moscow 119991, Russia c Institute of Organoelement Compounds Russian Academy of Science, Vavilova str. 28, Moscow 119991, Russia b

Abstract New hybrid materials based on nanocrystalline tin dioxide and two types of surface-immobilized polymer organosilicon structures with hydrocarbon substitutes were synthesized for gas sensors application. The sensing responses of pure SnO2 and hybrid samples were determined in the presence of NO2 (ppb range), CO (ppm range) and different humidity (RH = 15 – 95 %). Also the influence of water presence on sensor signal towards NO2 and CO was analyzed. Strong influence of nature of hydrocarbon substitutes on sensor response value towards NO2 and H2O was discovered. Keywords: Hybrid materials for gas sensors; Tin dioxide; Silazane

1. Introduction Hybrid organic-inorganic nanomaterials represent the new class of materials in which organic and inorganic moieties co-exist within a single matter. These materials have been extensively studied over the last few years due to possibility of improving functionality of individual components1. The interest to the hybrid materials is caused by a large number of available chemical and structural modifications. For example, hybridization by organics of nanocrystalline semiconducting oxides surface allows to manage selectivity and sensitivity of resistive gas sensors based on these oxides2. One of research directions on hybrid materials is to produce a cover on semiconducting oxides surface for selective separation of target gas molecules from interfering impurities. The requirements for the cover materials are thermal stability, uniformity of matrix structure and presence of scalable moieties which could be modified for regulation of selectivity of separation throughout the cover. This work is aimed to synthesize new hybrid materials for gas sensors based on the nanocrystalline tin dioxide and a surface-immobilized polymer organosilicon structure with hydrocarbon substitutes. Two types of silazanes such as polyvinyldimethylsilazane (Silazane1) and iso-nonylsilazane (Silazane2) (fig. 1) were chosen as monomers. The different hydrocarbon silazane sidechains were chosen to solve the problem of humidity influence on sensor response.

* Corresponding author. Tel.: +7-495-939-54-71; fax: +7-495-939-09-98. E-mail address: [email protected].

1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.043

E.A. Makeeva et al. / Procedia Chemistry 1 (2009) 17 2–17 5

(a) H2N

173

Table 1. List of samples. CH=CH2 CH=CH2 CH3 N Si H Si N Si N H H H HN NH CH3 Si CH=CH2 H2N m

(b)

iso-C9H19 H2N

Si

N H

H

NH2 n

Fig.1. Structures of organosilicon modificators: (a) polyvinyldimethylsilazane (Silazane1), (b) iso-nonylsilazane (Silazane2).

Sample

mass. % of silazane

Note

SnO2

-

Reference sample

MSnO2

-

Commercial available tin dioxide (Merck)

SnO2 -SiN1

2.2 %

On-chip film modification

SnO2 -SiN2

2.2 %

On-chip film modification

SnO2 -SiN1-p

0.22 %

Powder modification

SnO2 -SiN2-p

0.22 %

Powder modification

MSnO2-SiN1-p

2.2 %

Merck powder modification

MSnO2-SiN2-p

2.2 %

Merck powder modification

2. Experimental The nanocrystalline tin dioxide was prepared by the standard wet-precipitation method with subsequent heating at 300°C3. The hybrid samples were produced by two ways. In first series the initial tin dioxide powders were impregnated by silazanes solutions in toluene (SnO2-SiN1-p and SnO2-SiN2-p samples, table 1). Then the hybridized powders were deposited in the thick films form on microelectronic chips with interdigital Pt electrodes and Pt meander as heater. In second series the on-chip films of pure SnO2 were modified by appropriate silazane solution (SnO2-SiN1 and SnO2-SiN2 samples, table 1). Finally, all the chips were kept in stream of moist air at 150°C to form the polymer structure on SnO2 surface. The chip with unmodified SnO2 was made as a reference sample. The reference samples for spectroscopic researches (MSnO2, MSnO2-SiN1-p and MSnO2-SiN2-p samples, table 1) were produced based on commercial available tin dioxide (Merck, 99.0 %) by first method. The infrared (IR) spectra of these samples were recorded at room temperature with a Spectrum One (Perkin-Elmer) spectrometer in the diffuse reflectance mode between 370 and 4000 cm-1. The IR spectra of initial silazanes were recorded in attenuated total internal reflection mode between 370 and 4000 cm-1. Thermal gravimetric analysis of SnO2, SnO2-SiN1-p and SnO2-SiN2-p samples was performed from room temperature and up to 600°C in a thermal balance (STA 409 PC/PG, Netzsch). The heating rate was 10 K min-1. To investigate the sensor properties of hybrid materials, in situ conductivity measurements were carried out in an automated cell at a stabilized voltage. The sensor responses of SnO2-SiN1 and SnO2-SiN2 hybrid samples and of pure SnO2 sample toward NO2 (30 - 170 ppb range) and CO (300 - 600 ppm range) were determined at 230°C in dry air (RH = 4 %) and in humid air (RH = 95 %). In addition, the sensor properties of all the samples towards humidity were investigated at 200°C with humidity range 15 – 95 %. 3. Results and discursion The IR spectra of pure silazanes as well as MSnO2-SiN1-p and MSnO2-SiN2-p have a number of characteristic bands in 2700-3100 cm-1 range (fig. 2). These bands have good correlation with literature data for C-H bonds stretching as shown in Table 2. While IR spectrum of pure MSnO2 hasn't any peaks in this range. This confirms the presence of the silazanes on surface of the hybridized samples. The results of thermogravimetric and differential scanning calorimetry indicate that thermal decomposition of the silazanes starts in the temperature range 280-350°C. This allows us to select the operating temperature for gas sensor measurements. In the presence of NO2 and CO, the stable and well reproducible sensor signal was obtained for all the investigated samples (fig. 3a). In dry air (RH = 4 %, fig. 3b) the surface hybridization by iso-nonylsilazane (Silazane2) results in ten times increase of sensor response towards NO2, especially in low concentrations area (<100 ppb). While the surface immobilization of polyvinyldimethylsilazane (Silazane1) yields to no significant differences compared to the

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reference sample of pure SnO2. In humid air (RH = 95 %, fig. 3c) the sensor response of all the samples was decreased in comparison with dry air environment. Nevertheless the hybridization by Silazane2 results to increase of SnO2 sensor response towards NO2, while the hybridization by Silazane1 gives the reverse effect. (a)

(b)

Fig. 2. (a) IR spectra of (1) MSnO2, (2) MSnO2 -SiN1-p, (3) MSnO2 -SiN2-p in the 2700-3100 cm-1 range; (b) IR spectra of (1) Silazane1 and (2) Silazane2 in the 2700-3100 cm-1 range. Table 2. Characteristic infrared bands of MSnO2-SiN1-p and MSnO2 -SiN2-p samples and silazanes in 2700-3100 cm-1 range. MSnO2-SiN1-p

MSnO2-SiN2-p

Silazane1

3062

3047

3022

3007

Attribution4

Silazane2 3100–3000

=C–H stretching (vinyl moiety)

2959

2959

2956

2956

2960

Methyl symmetric C–H stretching

2925

2927

2945

2924

2930

Methylene asymmetric C–H stretching

2854

2874

2898

2871

2870

Methyl asymmetric C–H stretching

(a)

(b)

(c)

Fig. 3. (a) Typical conductivity plots during the cyclic change of gas phase composition at 230°С with RH = 95 %; (b) sensitivity toward NO2 at 230°С in dry air (RH = 4 %); (c) sensitivity toward NO2 at 230°С with RH = 95 %. The analyzed samples are (1) pure SnO2, (2) SnO2-SiN1 and (3) SnO2-SiN2.

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(a)

(b)

175

(c)

Fig. 4. (a) Sensitivity toward CO at 230°С in dry air (RH = 4 %); (b) sensitivity toward CO at 230° with RH = 95 %; (c) sensitivity toward H2O at 200°С. The analyzed samples are (1) pure SnO2, (2) SnO2 -SiN1, (3) SnO2-SiN2, (4) SnO2 -SiN1-p and (5) SnO2 -SiN2-p.

Though, no significant influence of polysilazane hybridization on the sensor responses toward CO (fig. 4a, 4b) was observed. In all cases the silazanes surface hybridization reduces the sensor response of SnO2 towards H2O. It is important to note two tendencies:  the hybridization by the more hydrophobic Silazane2 lead to greater decrease of sensor response then the hybridization by silazane with small hydrocarbon substitutes Silazane1;  to decrease the sensor sensitivity to humidity, the hybridization of tin dioxide powder is more effective then the hybridization of the pure SnO2 thick films on the chips (fig. 4c). Thus, the preliminary hybridization of tin dioxide powder by organosilicon compounds yields to better hydrophobic barrier than the modification of formed on microelectronic chip SnO2 thick film. 4. Conclusions New hybrid materials based on nanocrystalline tin dioxide and two types of surface-immobilized polymer organosilicon structures with hydrocarbon substitutes were synthesized by impregnation of tin dioxide powder and by modification of on-chip films of pure SnO2. The sensing responses were determined in the presence of NO2 (ppb range), CO (ppm range) and different humidity (RH = 15 – 95 %). In all cases the silazanes surface hybridization reduces the sensor response of SnO2 towards H2O, the preliminary hybridization of powder yields to better hydrophobic barrier. The hybridization by Silazane2 results to increase of SnO2 sensor response towards NO2 in low concentrations area (<100 ppb) both in dry and in humid air. The observed differences could be discussed in terms of matrix effect on H2O prefiltering near semiconductor oxide surface hybridized by Silazane2 with more hydrophobic substitutes. Thus, some of the obtained hybrid materials could be used for selective determination of nitrogen dioxide in a real humid environment.

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